11beta-Hydroxylase deficiency (11beta-OHD) inherited in an autosomal recessive manner accounts for about 5-8% of congenital adrenal hyperplasia (CAH). In order to clarify the underlying mechanism causing 11beta-OHD, we have done the molecular genetic analysis on the CYP11B1 gene in a patient diagnosed as 11beta-OHD. The nucleotide sequence of the patient's CYP11B1 revealed a novel nonsense mutation that converts codon 265 CAG (glutamine) to TAG (stop) of exon 4. Restriction fragment length polymorphism (RFLP) data showed that the patient was homozygous for the mutation. The above results confirm that the patient suffers from complete loss of the final step in cortisol biosynthesis pathway because of the nonsense mutation.